Farnesoid X receptor represses hepatic human APOA gene expression
Indumathi Chennamsetty, Thierry Claudel, Karam M. Kostner, Anna Baghdasaryan, Dagmar Kratky, Sanja Levak-Frank, Sasa Frank, Frank J. Gonzalez, Michael Trauner, Gert M. Kostner
Indumathi Chennamsetty, Thierry Claudel, Karam M. Kostner, Anna Baghdasaryan, Dagmar Kratky, Sanja Levak-Frank, Sasa Frank, Frank J. Gonzalez, Michael Trauner, Gert M. Kostner
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Research Article Genetics

Farnesoid X receptor represses hepatic human APOA gene expression

Abstract

High plasma concentrations of lipoprotein(a) [Lp(a), which is encoded by the APOA gene] increase an individual’s risk of developing diseases, such as coronary artery diseases, restenosis, and stroke. Unfortunately, increased Lp(a) levels are minimally influenced by dietary changes or drug treatment. Further, the development of Lp(a)-specific medications has been hampered by limited knowledge of Lp(a) metabolism. In this study, we identified patients suffering from biliary obstructions with very low plasma Lp(a) concentrations that rise substantially after surgical intervention. Consistent with this, common bile duct ligation in mice transgenic for human APOA (tg-APOA mice) lowered plasma concentrations and hepatic expression of APOA. To test whether farnesoid X receptor (FXR), which is activated by bile acids, was responsible for the low plasma Lp(a) levels in cholestatic patients and mice, we treated tg-APOA and tg-APOA/Fxr–/– mice with cholic acid. FXR activation markedly reduced plasma concentrations and hepatic expression of human APOA in tg-APOA mice but not in tg-APOA/Fxr–/– mice. Incubation of primary hepatocytes from tg-APOA mice with bile acids dose dependently downregulated APOA expression. Further analysis determined that the direct repeat 1 element between nucleotides –826 and –814 of the APOA promoter functioned as a negative FXR response element. This motif is also bound by hepatocyte nuclear factor 4α (HNF4α), which promotes APOA transcription, and FXR was shown to compete with HNF4α for binding to this motif. These findings may have important implications in the development of Lp(a)-lowering medications.

Authors

Indumathi Chennamsetty, Thierry Claudel, Karam M. Kostner, Anna Baghdasaryan, Dagmar Kratky, Sanja Levak-Frank, Sasa Frank, Frank J. Gonzalez, Michael Trauner, Gert M. Kostner

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Figure 4

FXR agonists downregulate APOA gene expression in a dose- and time-dependent manner in primary mouse hepatocytes.

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FXR agonists downregulate APOA gene expression in a dose- and time-depen...
(A) Primary mouse hepatocytes from tg-APOA mice were incubated with increasing concentrations of CA (50, 100, and 200 μM) or vehicle (control) for 24 hours. APOA mRNA levels were analyzed by real-time quantitative PCR. These data are presented as mean ± SEM (**P ≤ 0.01, *P < 0.05). (B) Primary mouse hepatocytes were incubated with CA (200 μM) or vehicle for 12, 24, and 48 hours. APOA mRNA levels were measured by real-time quantitative PCR. Results represent mean ± SEM of 3 independent experiments (**P ≤ 0.01). (C) Western blotting and densitometric analyses of APOA expression in whole cell lysates from hepatocytes treated for 24 hours with increasing concentrations of CA (expressed as mean ± SD relative to controls; *P < 0.05). (D) Primary hepatocytes were treated with GW4064 (5 μM) for 24 hours and analyzed for APOA mRNA levels by real-time quantitative PCR. These data are presented as mean ± SEM of 3 independent experiments (**P ≤ 0.01). (E) Western blotting and densitometric analyses of APOA expression in whole cell lysates from hepatocytes treated for 24 hours with GW4064 (5 μM) (expressed as mean ± SD relative to controls; **P ≤ 0.01).